1. Field of the Invention
This invention relates to solar shades for skylights, and more particularly relates to an energy-saving skylight cover system using one or more of diffraction gratings, angle-selective light transmitting and reflecting surfaces, and spectrally selective materials, to provide automatic control of the entry of selected radiation components of sunlight, notably ultraviolet, infrared and visible light, into an interior space.
2. Statement of the Problem
Many residential and commercial buildings have skylights to permit entry of daylight into their interiors. Most of these skylights are mounted on roofs that are not horizontal. Most of the skylights are also of a permanent nature, i. e., they are not openable. They are extremely useful devices, creating a window on the ceiling of the building and letting in much desirable daylight.
However, there are several aspects of skylights that are not desirable. Since most of the skylights consist of a clear glass pane mounted in a suitable frame, they permit entry of direct sunlight into the interior. Whereas during winter months such additional sunlight entering a room is desirable, in summer months the direct sunlight is more of a problem than an advantage. Direct sunlight in summer significantly increases heating of the interior space, making it more expensive to cool the room. Direct sunlight falling on furniture, upholstery, rugs, artwork and other contents of the room also causes their discoloration, fading, and faster overall deterioration. Direct sunlight may also be undesirable for various indoor plants and items such as books and art objects in shelves. Interior wall paint, particularly if other than white, is also known to fade and discolor more rapidly if exposed to direct sunlight. In summary, it would be highly desirable to be able to reduce the sunlight entering a room through a skylight in summer months without sacrificing the skylight's benefits including its aesthetic appearance.
3. Description of Related Art
A wide variety of window and skylight techniques are used for daylight entry, glare control, summer solar heat gain control, and winter heat loss control in residential and commercial buildings. These include:
a. fixed and adjustable mechanical methods such as shades, blinds, fins, awnings, slats and louvers; PA1 b. conventional optical techniques such as use of colored low-transmittance glasses; PA1 c. spectrally selective coatings of single metal layers and multilayer dielectric-metal-dielectric stacks; and PA1 d. chromogenic structures based on photochromic, thermochromic and electrochromic phenomena. PA1 a. venetian blinds mar the inherent aesthetic beauty that skylights provide to the interior of a room; and PA1 b. they block the sunlight after it has entered the room, thereby preventing only the light from entering the room, but not the heat. PA1 a. It should admit adequate amount of daylight; PA1 b. It should enhance architectural beauty of the building interior; PA1 c. It should act as a thermal barrier for interior heat and provide for solar heat gain in winter; PA1 d. It should minimize solar heat gain in summer; and PA1 e. It should provide spectral control to enable selection and rejection of different sunlight components as desired. PA1 Halliday & Resnick, PHYSICS, Wiley & Sons, N.Y., 1966, pages 1123-1124. PA1 Hecht & Resnick, OPTICS, Addison-Wesley, 1974, pages 354-358. PA1 Born & Wolf, PRINCIPLES OF OPTICS, Pergamon Press, 1986, pages 401-404. PA1 (a) Fixed exterior sun control devices such as awnings and overhangs are inherently incompatible with skylights because of the latter's conventionally non-vertical orientation and the concomitant installation difficulties. PA1 (b) Movable exterior systems, e. g., fins, slats and louvers are similarly inappropriate for skylights due to installation difficulties. They are high in installation and service costs, and also have the disadvantage of being susceptible to damage from adverse weather conditions such as snow, rain, frost, and high winds. PA1 (c) Movable interior mechanical systems such as shades, blinds and drapes are also inconvenient to install on skylights, and when used, mar the inherent architectural beauty added by the skylight to the interior. In addition, they do not offer selectivity between the visible and infrared portions of the solar spectrum. PA1 (d) Low-transmittance glass skylights, while reducing the cooling loads in the summer months, also permanently limit the available daylight. In winter months, when the solar heat gain is desirable, the low-transmittance feature cannot be deactivated. PA1 (e) Spectrally selective coatings designed for high transmittance in the visible portion of the solar spectrum and high reflectance in the infrared portion make a skylight efficient in summer months, but do not provide effective utilization of the solar heat gain in winter. PA1 (f) Chromogenic (photochromic, thermochromic, and electrochromic) coatings provide dynamic, but limited and non-user-modifiable, control of solar heat gain and luminous transmittance. Further, for large-scale application to skylights, economical chromogenic coatings with satisfactory performance are not yet available. PA1 (g) Energy-efficient window systems employing fixed and movable diffraction grating panes according to the earlier invention of coinventor Dr. Kanti Jain, referenced above, are effective but relatively expensive and complex for use as skylights.
Most of these existing methods, however, are ineffective, inconvenient, or too expensive for use with skylights. One method that is sometimes used on skylights to cut down their sunlight transmission is venetian blinds. The disadvantages of venetian blinds on skylights are twofold:
Therefore, the need persists for new means and methods that can make skylights energy-efficient and radiation protective without diminishing their benefits of providing daylight and enhancing comfort and beauty of the interior space.
Energy-Efficient Skylights: General Considerations
An energy-efficient skylight should be capable of controlling the transfer of energy in both directions according to desired criteria. In the most basic terms, clearly, a good skylight should provide to the building occupants both optical and thermal comfort while minimizing expended energy. Overall, an optimized skylight should have the following characteristics:
Available Methods for Improving Window Efficiency and Their Applicability to Skylights
A wide variety of techniques have been developed and employed for controlling energy transfer through windows. These include various mechanical, thermal and optical methods. The most common of these are movable interior sun control devices such as shades, drapes, blinds, etc. However, the above techniques are only marginally able to contribute in improving the energy efficiency of skylights because the skylights are not in a vertical plane and face the sun more directly. Fixed exterior sun control systems for windows, such as awnings and overhangs, are totally inapplicable to skylights.
Traditional Optical Methods
Many different optical techniques have been developed for window efficiency improvement. Some of these techniques are more readily applicable to skylights than others. Low-transmittance glass is widely used in windows and sometimes also in skylights. The glass is highly colored and is made light-absorbing by various additives. This helps in reducing the cooling load in summer months, but simultaneously, it also limits the available daylight substantially and reduces the beneficial solar heat gain in winter months.
Since the visible and infrared components of solar radiation are partially separable, it is possible to coat the windowpanes so that they transmit the solar luminous radiation (.lambda..about.0.4-0.7 .mu.m) while blocking the solar infrared spectrum (.lambda..about.0.7-3.0 .mu.m). Since the distribution of solar radiation among the above two spectral regions is nearly equal, in principle it is possible to prevent approximately half of the solar energy from entering the interior without impacting the daylight transmittance of the window. Such spectrally selective coatings are typically thin layers of a free-electron metal, such as Cu, Ag or Au. The luminous transmittance can be boosted by sandwiching the metal layer between two layers of high-refractive-index dielectric materials. Commercially, such coatings are now applied widely to vast numbers of windows and to some extent, to skylights.
Diffraction Grating
A diffraction grating is an arrangement which imposes on incident light a periodic variation of amplitude or phase, or both. A typical diffraction grating is a plastic sheet with a number of equidistant parallel depressions which cause characteristic spectral dispersion of transmitted light.
Diffraction gratings are well known, and are available from a number of sources including the following:
______________________________________ Acton Research Corp. Milton Roy Co. Optometrics USA Inc. ______________________________________ Acton MA Ayer MAer NY ______________________________________
Typical discussions of diffraction gratings appear in the following textbooks:
The angle-selective radiation controlling windowpane (16) may be a multilayer dielectric coating on a suitable substrate. Substrates generally are temporary or permanent support layers for other layers, which typically cannot support themselves.
The sun's incidence angle is the angle between sun and horizon.
The skylight characteristic angle is the angle designed into a particular skylight, related to its orientation on a roof, including the angle of the roof and the geographical location of the building, together with its built-in optimizations of rejection or acceptance of solar heat gain.
A radiation scupper is a device which accepts solar radiation and redirects it for discharge or reuse.
Note that whereas spectrally coated windows can exhibit significantly more summertime energy efficiency than uncoated glass, they will still fall short of the best achievable performance because they fail to adapt to seasonal changes and different climatic conditions. In cold climates it is also important to provide a good thermal barrier to prevent loss of interior heat through the windows. This can be done by using evacuated dual-pane glazings, and by using dielectric-metal-dielectric glazings optimized to provide high reflectivity at longer wavelengths (3-50 .mu.m spectral region). Window coatings for good thermal insulation and good solar luminous transmittance have also been made by applying thin layers of certain heavily doped oxide semiconductors, such as SnO.sub.2 :F, In.sub.2 O.sub.3 :Sn and ZnO:A2. It should be noted that as additional coatings become necessary to achieve transmittance control of different solar spectral regions, the window cost increases significantly. For these reasons, such complex coatings are not yet commonly found in windows, and much less in skylights.
Chromogenic Coatings
The most desirable function in an ideal window or skylight is dynamic control of heat gain, heat loss, and luminous transmittance as a function of varying conditions during the day or with the seasons. Many types of such `smart` coatings have been developed for windows and fall under the broad category of chromogenic coatings. These include photochromic, thermochromic and electrochromic coatings. Photochromic coatings undergo change in their transmittance properties as the intensity of the radiation incident on them changes. Photochromic sunglasses are a well known example of such a coating. The optical properties of thermochromic coatings are determined by temperature changes. Electrochromic layers use the phenomenon of electrically-activated injection or extraction of mobile ions into or from a certain region. It is well known in oxides of various transition metals such as W, V, Mo, Ni, Ti, Ir, etc. and many organic materials, and enables one to vary radiation transmittance over a wide range, e. g., 20-70%. However, for large-scale application to windows and skylights, economical chromogenic coatings with satisfactory performance (i. e., a full dynamic range of optical and thermal control) are not yet available.
The substrate may be the energy reflecting windowpane (16) diffraction grating.
The preferred diffraction grating is selective for accepting low azimuth angle sunlight and rejecting high azimuth angle sunlight. Where the energy reflecting windowpane (16) comprises a plurality of layers, in optical series, at least one of the layers should be a diffraction grating and at least one of the layers may be a chromogenic coating.
Energ-Efficient Windows with Diffraction Gratings and Scuppers
In a copending patent application, one of the coinventors of this invention, Dr. Kanti Jain, describes an energy-efficient window system with multiple fixed and movable diffraction grating windowpanes. These fixed and movable panes are either positioned to admit sunlight into the interior space of a room, or alternatively positioned to direct certain sunlight components to a set of scuppers which can absorb heat for use or disposal, or are aligned to reflect heat back out of the interior space. See United States patent application of Kanti Jain, Ser. No. 08/047,238, filed Apr. 13, 1993, ENERGY EFFICIENT WINDOW.